The present invention relates to a new class of polymeric materials that generate exceptionally high dielectric constant and high electromechanical response at ambient temperature. More particularly, the invention relates to a class of semicrystalline ferroelectric terpolymers comprising vinylidene fluoride (VDF), trifluoroethylene (TrFE), and at least one bulky monomer, such as chlorotrifluoroethylene (CTFE) or hexafluoropropene (HFP) or the like, prepared by borane/oxygen initiation in bulk reaction conditions.
Ferroelectric materials that generate mechanical actuation induced by external electric field have attracted a great deal of attention and have been recognized for applications in a variety of transducers, actuators and sensors. Most of the current commercial applications for ferroelectric materials are based on piezoceramics and magnetostrictive materials, despite the fact that they exhibit many deficiencies, such as low strain levels ( less than 0.2%), brittleness, heavy weight, high processing temperatures and processing difficulties when producing parts having complicated shapes. In sharp contrast, ferroelectric polymers exhibit many desirable properties, such as flexibility, light weight, high mechanical strength, an ability to be processed readily into large area films, and an ability to be molded readily into a variety of configurations. However, despite these advantages over ceramic materials, most ferroelectric polymers suffer the disadvantage of having a low electric field sensitivity, in terms of dielectric constant, piezoelectric coefficient, electromechanical coupling coefficient and field induced strain, which limit their applications.
One of the phenomena in ferroelectric polymers that has a great potential in generating high strain with high force level and broad frequency bandwidth is the phase transformation between ferroelectric (polar) and paraelectric (nonpolar) crystalline domains. The crystalline phase change produces large lattice strain and large change in sample dimension. Electrostriction refers to a coupling effect between the strain and the square of polarization, and is a desirable mechanism for achieving a large electric-induced mechanical response. It is interesting to note that the electrostrictive response due to crystalline phase transition is very different from electrostatic force in dielectric elastomers [R. Pelrine, R. Kornbluh, Q. Pei, and J. Joseph, Science, 287, 836, 2000], which can produce a large strain but a very weak force.
In the past decade, most of the research activities involving ferroelectric polymers have focussed on ferroelectric fluorocarbon polymers, especially semicrystalline VDF/TrFE copolymers. Many research efforts have been devoted to a general goal of reducing the energy barrier for ferroelectric-paraelectric (Curie) phase transition, and for generating a large and fast electric-induced mechanical (piezoelectric) response at ambient temperature. Although VDF/TrFE copolymers (stretched film poled at 120xc2x0 C.) exhibit a relatively high piezoelectric constant (on the order of from about 10 pC/N to about 25 pC/N; pC=10xe2x88x9212 coulomb and N=newton) [K. Koga, and H. Ohigashi, J. Appl. Phys., 59, 2142,1986], the response of the dipoles to an electric field is very slow at ambient temperature, and the polarization hysteresis loop (polarity vs. electric field) of the copolymer is very large. As shown in FIG. 3, a VDF/TrFE copolymer comprising 55 mole % VDF and 45 mole % TrFE, which exhibits the narrowest polarization hysteresis loop and lowest Curie temperature of the copolymers in the VDF/TrEF family [Y. Higashihata, J. Sako, and T. Yagi, Ferroelectrics, 32, 85, 1981], still exhibits a significantly wider hysteresis loop than those exhibited by the VDF/TrEF/bulky monomer terpolymers of this invention.
The close connection between crystalline structure and electric properties led to many attempts to alter copolymer morphology by creating non-equilibrium states; and a number of such attempts resulted in ferroelectric polymers that exhibit somewhat improved electric responses. Such attempts have included, for example, subjecting ferroelectric polymers to mechanical deformation [K. Tashiro, S. Nishimura, and M. Kobayashi, Macromolecules, 21, 2463, 1988, and 23, 2802, 1990], electron-radiation [B. Daudin, and M. Dubus, J. Appl. Phys., 62, 994, 1987], uniaxial drawing [T. Furukawa, and N. Seo, Japanese Journal of Applied Physics, 29, 675, 1990], crystallization under high pressure [T. Yuki, S. Ito, T. Koda, and S. Ikeda, Jpn. J. Appl. Phys., 37, 5372, 1998], and crystallization under high electric field [S. Ikeda, H. Suzaki, and S. Nakami, Jpn. J. Appl. Phys., 31, 1112, 1992].
Zhang et al. [Science, 280, 2102, 1998 and WO99/26261] recently reported their work involving electron-radiation treatment of ferroelectric polyvinylidene fluoride polymers. The polymers that were generically disclosed in their work include polyvinylidene fluoride, polyvinylidene fluoride-trifluoroethylene, polyvinylidene fluoride-tetrafluoroethylene, polyvinylidene fluoride-trifluoroethylene-hexafluoropropylene and polyvinylidene fluoride-hexafluoropropylene. However, the only polymers actually prepared and studied were 50/50 and 65/35 copolymers of vinylidene fluoride/trifluoroethylene (VDF-TrFE). The Zhang et al work, which included a systematic study of the radiation conditions, such as dosage, temperature, inert atmosphere, stretching sample, etc., revealed an exceptionally high electrostrictive response (xcx9c4%) of the irradiated copolymer that behaves like a relaxor ferroelectric with fast electric-induced mechanical response at ambient temperature. Their work also revealed that the polarization hysteresis loop of the irradiated copolymer became very narrow at room temperature, compared with the hysteresis loop of a sample of the copolymer before irradiation. However, the polarization of the irradiated copolymer also was significantly reduced and the irradiated copolymer became completely insoluble because of the severe crosslinking side reaction that had occurred during the high-energy radiation. The increase of hardness of the irradiated copolymer sample due to the crosslinking also was revealed in its electric response, i.e., a very high electric field (150 MV/m) was required before the irradiated copolymer exhibited a high strain response (xcx9c4%). Thus, it appears that irradiating a ferroelectric copolymer not only reduces the polar crystalline domain size, but also produces many undesirable side reactions that increase the amorphous phase and diminish the processibility of the irradiated copolymer. At page 11, lines 25-27 [in WO 99/26261], Zhang et al, in a quite off-handed manner, suggested that the effects achieved by irradiation can be accomplished chemically, by adding a bulky side group to the main polymer chain which operates as an internal plasticizer. Zhang et al provided no examples of bulky side chain additions and made no further reference to chemically modified polymers.
As will be apparent from the following description of the invention, the present approach for altering the crystalline domains, and creating relaxor ferroelectric behavior, of VDF/TrFE copolymers is to introduce into the copolymer structure a controlled amount of bulky monomer units, such as chlorotrifluroethylene (CTFE) and hexafluoropropene (HFP) units, with a homogeneous fashion. The resulting terpolymers are solution and melt processible and form a desirable film morphology with uniform nano-crystalline domains that have Curie (polar-nonpolar crystalline phase) transition at about ambient temperature. Therefore, these terpolymers exhibit exceptionally high dielectric constant at ambient temperature and fast and high electro-mechanical response induced by external electric field.
Prior to the present invention, there have been several reports that discussed VDF/TrFE/CTFE terpolymers with significantly different thermal and electric properties. The terpolymers were prepared by emulsion and suspension polymerization processes at elevated temperature. For example, U.S. Pat. No. 4,554,335 discloses the stable dielectric constants over a wide range of temperatures in the terpolymers that are composed of 25-90 mol % of VDF units, 5-70 mol % TrFE units, and 1-13 mol % CTFE units. However, these polymers have a dielectric constant of at most about 20 at ambient temperature (20xc2x0 C.) at 1 kHz. The stability of dielectric constants over a wide range of temperatures implies a terpolymer with a broad Curie (polar-nonpolar crystalline phase transition) temperature that is associated with a broad range of crystalline domains and a broad terpolymer composition distribution. U.S. Pat. No. 5,087,679 discloses VDF/TrFE/CTFE terpolymers having a TrFE content in the range of 18-22 mole %. The terpolymers disclosed in that patent exhibit a dielectric constant higher than 25 (but below than 40) at 20xc2x0 C. 1 kHz. The properties that are disclosed in these two patents are very different from those exhibited by the terpolymers of the present invention, in which the VDF/TrFE/CTFE terpolymers were prepared by bulk process at ambient temperature using borane/oxygen initiation. FIG. 1 compares the dielectric constant of a terpolymer sample Example 6 in this disclosure) with that of the best sample shown in the previous disclosures (U.S. Pat. No. 5,087,679). The dielectric constant of the sample in this disclosure is significantly higher over a wide temperature range, and the dielectric constant reaches about 100 at 20xc2x0 C. 1 kHz. In fact, this terpolymer (of Example 6) has a mole ratio of VDF/TrFE/CTFE=58/33.1/8.9, which is well off the best composition suggested in the previous disclosure (U.S. Pat. No. 5,087,679). In addition, the dielectric constant remains very high over a wide frequency range (650 Hz to 300 kHz), as shown in FIG. 2.
There also have been reports that discuss VDF/TrFE/HFP terpolymers containing bulky hexafluoropropene (HFP) monomer units. For example, Freimuth et al. [H. Freimuth, C. Sinn, and M. Dettenmaier, Polymer, 37, 831, 1996] disclose that the incorporation of HFP into VDF/TrFE copolymer did not affect the crystalline structure, but strongly reduced the degree of crystallinity of the resulting polymer. Sako et al. [J. Sako, T. Yagi, Y. Higashihata, M. Tatemoto, N. Tomihashi, Y. Shimizu, U.S. Pat. No. 4,577,005] reported improved specific permittivity (dielectric constant) for VDF/TFE/HFP terpolymers, compared to VDFTFE copolymers. However, the dielectric constant of the terpolymers remained low ( less than 20) at room temperature and 1 kHz, even after heat treatment. Tajitsu et al. [Y. Tajitsu, A. Hirooka, A. Yamagish, and M. Date, Jpn. J. Appl. Phys., 36, 6114, 1997] reported a switching phenomenon in the VDF/TrFE/HFP terpolymers having a low HFP content ( less than 2.5 mole %). The switching phenomenon is associated with the rotations of individual molecular chains around their axes in crystalline domains. Overall, the switching time was found to have a very low dependence on the HFP content of the terpolymer. With an increase in the HFP content, both polarization and dielectric constant were found to decrease at the Curie temperature, which usually is above 40xc2x0 C.
To date, methods of preparing VDF/TrFE/CTFE and VDF/TrFE/HFP copolymers have included free radical emulsion and suspension polymerization processes in aqueous solution using a batch reactor [F. J. Honn, et al., U.S. Pat. No. 3,053,818; J. E. Dohany, et al., U.S. Pat. No. 3,790,540; T. Sakagami, et al., U.S. Pat. No. 4,554,335; and H. Inukai, et al., U.S. Pat. No. 5,087,679]. The combination of heterogeneous reaction conditions, significant difference in comonomer reactivity ratios, and high monomer conversion in batch reactions inevitably results in terpolymers having a broad composition distribution and inhomogeneous crystalline domains. In addition, it is also very difficult to completely remove emulsifying and suspending agents (containing polar groups) after emulsion and suspension polymerization processes, respectively.
It is an object of the present invention to provide a new class of ferroelectric terpolymers, which exhibit an exceptionally high dielectric constant and large strain response under the influence of an electric field at ambient temperature.
It is another object to provide a new class of terpolymers, which are both melt and solution processible, and which possess a combination of uniform molecular and nano-crystalline structures, so that they have a crystal phase (polar-nonpolar) transition temperature (Curie temperature) at near ambient temperature and exhibit typical relaxor ferroelectric behavior.
Yet another object is to provide a process for preparing the subject ferroelectric terpolymers, which process utilizes homogeneous reaction conditions involving only the various monomers and a very small amount of borane and oxygen, which, in situ, form a borane/oxygen free radical initiator.
The above and other objects and advantages of the invention are achieved by providing easily processible (by solution or melt) semicrystalline terpolymers, which comprises 50-80 mole % of vinylidene fluoride (VDF), 15-40 mole % of trifluoroethylene (TrFE) and 2-20 mole % of at least one bulky monomer, such as chlorotrifluoroethylene (CTFE) or hexafluoropropene (HFP). The terpolymers have a number average molecular weight in excess of about 10,000, and preferably in excess of about 30,000. Typically, the molecular weight of the terpolymers would be on the order of from about 10,000 to about 500,000, and preferably from about 30,000 to about 100,000.
The terpolymers exhibit a high dielectric constant and a crystal phase (polar-nonpolar) transition temperature (Curie temperature) near ambient temperature. Typically, the dielectric constant of the present terpolymers would be at least about 40, and preferably at least about 50, e.g., on the order of from about 40 to about 100, and preferably from about 50 to about 100. The Curie temperature of the terpolymers typically would be from about 15xc2x0 C. to about 40xc2x0 C., and preferably from about 18xc2x0 C. to about 35xc2x0 C.
The terpolymers also exhibit an exceptionally large electrostrictive strain response under an external field at ambient temperature. As used in this specification and claims, the term xe2x80x9cambient temperaturexe2x80x9d is understood to be at 1 atmosphere and room temperature between 20-25xc2x0 C. Also, the term xe2x80x9cexceptionally large strain responsexe2x80x9d is understood to mean  greater than 0.5% strain at 50 MV/m and  greater than 2.5% at 100 MV/m.
As used in this specification and claims, the term xe2x80x9cterpolymerxe2x80x9d is meant to include not only polymers that contain units derived from three distinct monomers, but also polymers that contain units derived from greater than three monomers. For example, the term xe2x80x9cterpolymerxe2x80x9d includes polymers prepared from vinylidene fluoride, trifluoroethylene, and chlorotrifluoroethylene (VDF/TrFE/CTFE), as well as polymers prepared from vinylidene fluoride, trifluoroethylene, chlorotrifluoroethylene and hexafluoropropylene (VDF/TrFE/CTFE/HFP). In other words, the ferroelectric terpolymers of this invention include terpolymers of vinylidene fluoride, trifluoroethylene, and at least one bulky co-monomer, such as, chlorotrifluoroethylene (CTFE), hexafluoropropene (HFP), vinylidene chloride (VDC), tetrafluoroethylene (TFE), vinyl fluoride, vinyl chloride, acrylonitrile, acrylamide, methyl methacrylate, ethyl methacrylate, butyl methacrylate, octyl methacrylate, methacrylic acid, methyl acrylate, ethyl acrylate, butyl acrylate, octyl acrylate, 2-hydroxyethyl acrylate, glycidyl acrylate, acrylic acid, maleic anhydride, vinyl acetate, styrene, alpha-methyl styrene, trimethoxyvinylsilane, triethoxyvinylsilane and the like.
As used in this specification and claims the term xe2x80x9csubstantially uniform molecular and nano-crystalline structuresxe2x80x9d is understood to mean that the terpolymer under consideration has a narrow terpolymer composition that results in the terpolymer having uniform nano-crystalline domains. Both melting and Curie transitions in Differential Scanning Calorimetry (DSC) thermograms are strongly dependent on the terpolymer composition, with sharp melting peaks and diminished Curie (polar-nonpolar) phase transitions. The combination of significant reductions of both melting temperature and heat for polar-nonpolar crystal phase (Curie) transition indicates the terpolymers having very small crystalline domains. The crystalline phase was further examined by X-ray diffraction region for the (200) and (110) reflections. Only one narrow peak is observed in all terpolymers, and the peak systematically moves to the lower angle (2xcex8) as the CTFE content increases in a given terpolymer. The angle about 18.2xc2x0 (corresponding to a lattice spacing of 4.9 angstrom) for the terpolymer VDF/TrFE/CTFE =61.4/25.3/13.3 indicates the paraelectric phase in this terpolymer, which is different from the ferroelectric phase of the corresponding (VDF/TrFE=60/40) copolymer. The systematic increase of the lattice spacing clearly is due to the uniform terpolymer structure, and the uniformly distributed CTFE units serve as the defects in the crystalline phase.
The terpolymers of the invention are characterized by a unique combination of substantially uniform molecular and nano-crystalline structures, such that they exhibit typical relaxor ferroelectric behavior (with extremely low heat for polar-nonpolar crystalline phase transition). The expanding and contracting of these crystalline domains under an external electric field, coupled with a large difference in the lattice strain between the polar and non-polar crystal phases, generates the exceptionally large electrostrictive strain response that is characteristic of the terpolymers of this invention. The unique properties of the present terpolymers are believed to be due, at least in part, to the new chemistry that is used in their preparation, i.e., using oxidation adducts of an organoborane as a free radical initiator in a homogeneous bulk polymerization process to prepare semicrystalline terpolymers having a homogeneous molecular structure that results in uniform crystalline structure and desirable crystalline domains.